The source will be HDMI.
I believe that ProPresenter only supports a single video output. What they do for multiple screens is use the Matrox DualHead2Go or TripleHead2Go devices. So for your three screens the computer would basically be outputting one large 5760x1080 image that a Matrix TripleHead2Go would then split equally across three screens.
I also believe that the current versions of the TripleHead2Go support DVI-D, DisplayPort or Thunderbolt (and only support 1920x1080 at a 50Hz refresh rate, which the LHD700 does support). There is no HDMI version, in theory you should be able to use DVI-D to HDMI adapters and that likely would work, although I have found some HDMI input devices that do not work well with anything other than an actual HDMI signal. And if you might ever be displaying HDCP protected content then only the newer TripleHead2Go Digital SE version is HDCP compliant and it only supports Thunderbolt, Display Port and mini-DisplayPort.
Are you using the Multi-Screen module in ProPresenter? I believe that is where you would decide what is displayed on each screen.
ProPresenter supports masking but masking and scaling are different things so apparently what you would be doing for the 2.35:1 center image is inserting masking at the top and/or bottom of the center 1920x1080 third of the image so that you only see only a 1920x817 part of the image.
I definitely suggest talking to Renewed Vision about your application, they are usually very good in answering any questions. And also check out their support forums, many of your questions may already be addressed there.